Embodiments of the invention relate generally to a method for joining a ceramic component to a metal component. More particularly, the invention includes embodiments that relate to a ceramic-to-metal joining for sealing a high temperature electrochemical cell. The invention also includes embodiments that relate to a sealing structure formed by using such a method.
Many types of seal joints have been considered for use in high-temperature rechargeable batteries/cells for joining different components. Sodium/sulfur or sodium/metal halide cells generally include several ceramic and metal components. The ceramic components include an electrically insulating alpha-alumina collar and an ion-conductive electrolyte beta-alumina tube, and are generally joined or bonded via a sealing glass. The metal components include a metallic casing, current collector components, and other metallic components which are often joined by welding or thermal compression bonding (TCB).
A metal-to-ceramic bonding is most critical for the reliability and safety of the cell because it has several issues, mainly due to thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic and metal components. Many types of bonding materials and sealing processes have been considered for joining metal components to ceramic components, including ceramic adhesives, brazing, and diffusion bonding. However, most of the seals/joints may not withstand high temperatures and corrosive environments of the batteries/cells.
Current methods include metalizing the ceramic component, followed by bonding the metallized ceramic component to the metal component by thermal compression bonding. Usually, an outer surface of a metallization layer is further treated by plating the surface with a metal to form a metal layer on the metallization layer, before the metal component is bonded to the ceramic component. The plating layer provides a continuous metal layer on the metallization surface to obtain a high strength joint/bond with the metal component. Usually, although not necessarily, the plating metal is the same metal as that of the metal component, or a metal that is at least compatible with the metal of the metallization layer and with the metal component. For example, nickel has been conventionally used for plating a molybdenum-based metallization layer on alumina, specifically in high temperature batteries/cells.
Various methods have been used for plating metal (e.g., nickel) on the metallization layer, for example electroplating, electroless plating, screen printing, etc. Each of these processes has one or more issues with respect to the quality of the plating layer or the scalability of the process. Screen printing is a highly scalable, clean and cost effective process, but provides a porous metal layer. The porous plating layer may lead to the contamination of the metallization material which thus affects the strength of the resulting metal-to-ceramic bond/joint. This porous metal layer, for example, nickel layer, can be sintered (treated at high temperatures, e.g., above about 1200 degrees Celsius) for the densification of the layer, but may often form an intermetallic with the metal (e.g., molybdenum) of the metallization layer at high temperatures (usually above 1000 degrees Celsius). The formation of the intermetallic adversely affects the bonding between the plating layer and the metallization layer, and thus the joint/bond between the ceramic component and the metal component.
With these considerations in mind, it would therefore be desirable to develop new materials and methods for efficient ceramic-to-metal joining. Particularly, it may be desirable to have a method for sealing a high temperature electrochemical cell that differs from those methods that are currently available. It may be desirable to have a sealing structure that uses materials and methods that differ from those materials and methods that are currently available.